全文获取类型
收费全文 | 1138篇 |
免费 | 61篇 |
专业分类
林业 | 24篇 |
农学 | 33篇 |
基础科学 | 7篇 |
77篇 | |
综合类 | 197篇 |
农作物 | 30篇 |
水产渔业 | 24篇 |
畜牧兽医 | 747篇 |
园艺 | 5篇 |
植物保护 | 55篇 |
出版年
2021年 | 9篇 |
2020年 | 14篇 |
2019年 | 18篇 |
2018年 | 22篇 |
2017年 | 18篇 |
2016年 | 24篇 |
2015年 | 15篇 |
2014年 | 15篇 |
2013年 | 48篇 |
2012年 | 45篇 |
2011年 | 47篇 |
2010年 | 22篇 |
2009年 | 30篇 |
2008年 | 51篇 |
2007年 | 43篇 |
2006年 | 38篇 |
2005年 | 52篇 |
2004年 | 40篇 |
2003年 | 34篇 |
2002年 | 36篇 |
2001年 | 35篇 |
2000年 | 35篇 |
1999年 | 32篇 |
1998年 | 7篇 |
1997年 | 8篇 |
1996年 | 11篇 |
1995年 | 6篇 |
1994年 | 7篇 |
1993年 | 13篇 |
1992年 | 22篇 |
1991年 | 22篇 |
1990年 | 23篇 |
1989年 | 16篇 |
1988年 | 18篇 |
1987年 | 21篇 |
1986年 | 17篇 |
1985年 | 21篇 |
1984年 | 10篇 |
1983年 | 7篇 |
1982年 | 7篇 |
1980年 | 9篇 |
1979年 | 8篇 |
1978年 | 9篇 |
1977年 | 16篇 |
1974年 | 18篇 |
1973年 | 19篇 |
1972年 | 18篇 |
1971年 | 19篇 |
1970年 | 12篇 |
1966年 | 9篇 |
排序方式: 共有1199条查询结果,搜索用时 62 毫秒
81.
82.
Rapid-cycling populations of six economically important species in the genus Brassica have unusual potential for resolving many problems in plant biology and for use in education. Rapid-cycling brassicas can produce up to ten generations of seed per year and serve as models for research in genetics, host-parasite relations, molecular biology, cell biology, plant biochemistry, population biology, and plant breeding. Brassicas are a highly diverse group of crop plants that have great economic value as vegetables and as sources of condiment mustard, edible and industrial oil, animal fodder, and green manure. These plants can also be used in the classroom as convenient, rapidly responding, living plant materials for "hands on" learning at all levels of our educational system. 相似文献
83.
本文提出了测定水稻品种对环境稳定性的一个新参数,采用这一参数对美国加利福尼亚州多年多点的水稻品种整米率进行稳定性分析的结果表明,稳定性与熟期类型在统计上有一定的相关性,即整米率稳定性依晚熟品种、早熟品种、特早熟品种而顺序降低,回归分析说明整米率的稳定性有随整米率的提高而增加的趋向,本文还讨论了稳定性系数的优点和局限性。 相似文献
84.
K J Falke R D Hill J Qvist R C Schneider M Guppy G C Liggins P W Hochachka R E Elliott W M Zapol 《Science (New York, N.Y.)》1985,229(4713):556-558
Arterial blood nitrogen tensions of free-diving Weddell seals (Leptonychotes weddelli) were measured by attaching a microprocessor-controlled blood pump and drawing samples at depth to determine how these marine mammals dive to great depths and ascend rapidly without developing decompression sickness. Forty-seven samples of arterial blood were obtained from four Weddell seals during free dives lasting up to 23 minutes to depths of 230 meters beneath the sea ice of McMurdo Sound, Antarctica. Peak arterial blood nitrogen tensions of between 2000 and 2500 millimeters of mercury were recorded at depths of 40 to 80 meters during descent, indicating that the seal's lung collapses by 25 to 50 meters. Then arterial blood nitrogen tensions slowly decreased to about 1500 millimeters of mercury at the surface. In a single dive, alveolar collapse and redistribution of blood nitrogen allow the seal to avoid nitrogen narcosis and decompression sickness. 相似文献
85.
Hill HW 《Science (New York, N.Y.)》1924,59(1520):153-156
Public health is the science and art of conscious physical adjustment between man and his surroundings in the universe. The modern conception of man as a product of and a part of nature brings the subject of man's individual physical adjustments with his immediate surroundings into its proper place as the fundamental study-the basis of every form of education. Hence, public health is not only eligible for a position as an independent faculty in any university but is as definitely entitled to such a place as any of those now recognized. It is futile to consider the ordinary 45 hour course in public health, furnished as an incident in the ordinary 4000 to 5000 hour medical course, as more than a smattering, offered to medical students alone, of the 900 to 4500 hour courses in public health offered to professional public health students. 相似文献
86.
87.
Crystal structure of defensin HNP-3, an amphiphilic dimer: mechanisms of membrane permeabilization 总被引:34,自引:0,他引:34
Defensins (molecular weight 3500 to 4000) act in the mammalian immune response by permeabilizing the plasma membranes of a broad spectrum of target organisms, including bacteria, fungi, and enveloped viruses. The high-resolution crystal structure of defensin HNP-3 (1.9 angstrom resolution, R factor 0.19) reveals a dimeric beta sheet that has an architecture very different from other lytic peptides. The dimeric assembly suggests mechanisms by which defensins might bind to and permeabilize the lipid bilayer. 相似文献
88.
Welfare dependence within and across generations 总被引:2,自引:0,他引:2
A central question in the debate about the effects of welfare programs is whether their intended positive results--providing needed resources to recipients and their children--outweigh any unintended negative results--for example, the breaking up of families, reduction in work effort, or the fostering of a welfare "culture" that is passed on from parent to child. Recent research shows that although work effort is affected adversely by the generosity of welfare programs, effects on family structure appear quite weak, most welfare experiences are relatively short, and the majority of women who grew up in homes heavily dependent on welfare do not rely on those programs when they are young adults. 相似文献
89.
Muscle contraction and free energy transduction in biological systems 总被引:52,自引:0,他引:52
Muscle contraction occurs when the actin and myosin filaments in muscle are driven past each other by a cyclic interaction of adenosine triphosphate (ATP) and actin with cross-bridges that extend from myosin. Current biochemical studies suggest that, during each adenosine triphosphatase cycle, the myosin cross-bridge alternates between two main conformations, which differ markedly in their strength of binding to actin and in their overall structure. Binding of ATP to the cross-bridge induces the weak-binding conformation, whereas inorganic phosphate release returns the cross-bridge to the strong-binding conformation. This cross-bridge cycle is similar to the kinetic cycle that drives active transport and illustrates the general principles of free energy transduction by adenosine triphosphatase systems. 相似文献
90.
Heme-heme orientation and electron transfer kinetic behavior of multisite oxidation-reduction enzymes 总被引:1,自引:0,他引:1
Analysis of the polarized single-crystal absorption spectra of cytochrome cd1 of Pseudomonas aeruginosa shows that the heme c and heme d1 groups in each subunit are oriented perpendicularly to each other in both oxidized and reduced forms of the enzyme. These results, together with those of previous kinetic studies, indicate that a perpendicular heme-heme orientation may be an important factor in specifying kinetically slow steps in a sequential series of electron transfer reactions. 相似文献